This is in stark contrast to adults, 50% of whom will yawn if they see another person yawning, a phenomenon known as contagious yawning.

In the first part of the study, mothers were asked to record when and how often their children yawned. Babies and toddlers between the ages of 6 and 34 months yawned most after waking in the morning or after naps, but only yawned on average about twice a day.

This is much less than the average seven to nine times per day that an adult yawns.

The mothers did not report any contagious yawning in their children.

In the second part of the study, babies and toddlers with an average age of two years were shown videos of babies - some yawning and others not. Only three of the 22 children yawned when they saw a baby yawn on the video.

"The largely negative video results confirm that infants and preschoolers are much less susceptible to psychological influences on yawning when compared with older children and adults," say the authors.

Sign of empathy

Previous studies have shown that children under five years don't tend to contagiously yawn, but by the time they reach 12 years, they are doing so at the same rate as adults.

According to Anderson, this may be because contagious yawning has been shown in previous studies to be a sign of empathy, and children don't develop this trait until they are between three and six years old.

"Young children may also do less contagious yawning simply because they don't have the same pressures or social inhibitions as adults: They yawn where they like and when they like," he says.

Recent research also overturns the notion that we yawn because we're bored. "Yawning is a phylogenetically old behaviour, which is largely controlled by subcortical mechanisms," says Anderson. In other words, it's an ancient behaviour programmed into our DNA.

Studies have also shown yawning has important physiological and psychological functions. The sudden intake of cool air acts like a radiator, altering facial blood flow, cooling the brain and increasing alertness.

Anderson says contagious yawning might also have aroused our ancestors to respond quickly together to sudden danger. "Seeing someone else yawn might simply remind us to do the same thing," says Anderson.

Unnatural setting

Dr Mark Neilsen, an expert in early child development at the University of Queensland, questions the use of video in the study.

"We know that children at that age don't process that information from video in the same way they do as a live person," says Neilsen. "That's even more relevant when we're looking at a social stimulation, [such as yawning].

"They are making the claim that infants don't have contagious yawning in the same way as adults do, but they are using a medium of presentation that adults and infants react differently to. Children younger than two don't see TV as a social stimulus," he says.

But Anderson says informal experiments done by his team so far suggest that real live mothers yawning in front of their children are also ineffective.

"I'd like some more naturalistic data [a real person instead of a video] on contagious yawning, to see whether children are more likely to show contagious yawning at home, for example.

"Or perhaps we might see the origins of contagious yawning, especially when the infant is tired and the mother yawns."